Applications for accurate oscillation sources are innumerable. These include radios, radar, downed aircraft beacons and space borne apparatus, all of which may be subjected to significant temperature variations. It is well known that oscillators, and more specifically crystal oscillators, tend to vary in frequency with changes in ambient temperature. Frequency stability over wide varying temperatures is highly desirable and a number of methods have been devised for effecting such temperature compensation. One satisfactory method for large or relatively expensive installations is to provide an oven which can be very precisely maintained at a desired temperature so that there is substantially no temperature related frequency drift of the crystal oscillator. The amount of space and energy used in such an installation makes this method impractical for many applications.
Another common technique for minimizing the sensitivity of a crystal to temperature variations is to control the angle at which the crystal is cut with respect to its crystallographic axis because the temperature coefficient of a crystal is a function of the angle of cut. However, the degree to which the temperature coefficient may be reduced in this manner is quite limited in that the range of temperature over which this approach is effective is relatively small.
A third technique for frequency stabilization is electronic in character, temperature compensation being effected in an analog manner with the aid of a temperature-sensitive element. This results from the fact that the resonant frequency of a crystal may be varied by varying the magnitude of an external reactance connected in circuit with the crystal. Such methods essentially follow the procedure of sensing the temperature and developing a voltage functionally related to the crystal temperature. This voltage may be amplified and used to bias a voltage variable capacitor or other variable impedence means to pull the crystal to the desired frequency. Since the crystal frequency versus temperature curve is usually non-linear, and may not even be monotonic, the temperature-sensing voltage may require non-linear amplification in order to fit the frequency correction in a piecewise linear manner. Many temperature cycles and many man hours are used up in attempting to fit the circuit characteristics to a particular crystal to accomplish the desired compensation. One disadvantage of the analog control technique is that forcing a crystal to operate at a desired frequency which is unnatural for it at a particular temperature, often termed rubberizing, compromises the long-term stability of the crystal so that its actual frequency output at any particular temperature may not be stable and predictable over a long period of time. Thus the useful crystal life can be shortened by the rubberizing techniques.
Examples of analog compensation to affect the actual frequency of the crystal are disclosed in U.S. Pat. Nos. 3,404,297, 3,713,033, 3,397,367 and 3,719,838.
There are other techniques for obtaining temperature-compensated frequencies. One may be referred to as the variable modulus divider technique, examples of which are disclosed in U.S. Pat. Nos. 3,938,316 and 4,015,208. The circuits disclosed in these patents do not adjust the crystal oscillator frequency but provide other means for achieving the desired compensated frequency output. In the variable modulus divider circuits, the crystal oscillator frequency is divided by a relatively large number which is changed as a function of temperature in incremental steps.
Another patent which provides temperature compensation without adjusting the crystal oscillator is U.S. Pat. No. 4,159,622. This device employs two crystal oscillators at the same frequency. When temperature-caused phase difference occurs between the oscillators, a pulse is added to a divider to advance the frequency rate of the combined output.